Former Lord Mayor of Dublin Emer Costello is expected to replace Labour MEP Proinsias De Rossa when he steps down as an MEP next month.
Mr De Rossa gave no reason for his decision to resign from his post when he issued a statement in which he thanked those he had worked with.
He was first elected to the Dáil in 1982 and served as Minister for Social Welfare between 1994 and 1997, before being elected to the European Parliament in 1999.
Labour leader Eamon Gilmore paid tribute to him today, calling him a "great friend and colleague" and saying his reputation in Europe is that of "a passionate, hard-working and dedicated representative of Ireland".